Yesterday I pointed out a #33594 Truck that is currently for sale on eBay, but now it appears that it was actually part of a #33520 Trucks and Cars set, which is in itself also quite rare. The seller has listed the red, single-axel trailer from that set as a separate auction.
Usually when stuff like this happens—a seller splitting up a set or part of a set—it's because they aren't familiar with the product line and don't really know what they have or what goes with what. There are a lot of people selling BRIO on eBay, and a good chunk of them are probably parents getting rid of their child's old train set that has lost pieces over the years and was all mixed together and shoved into boxes. You can't blame people for not knowing which pieces are part of a particular set, or for breaking a large lot into more manageable pieces.
Usually. There are rare occasions—very rare—when it's just plain asshattery. Such as with this guy who sold the buildings from the Hospital set in one auction:
The vehicles in another:
And then the empty box three days later:
But most folks don't do this sort of thing and I tend to give sellers the benefit of the doubt. As frustrating as it may be to the collector, the truth is that most people just don't know what they have, or that they are breaking up a set. As a buyer, only you can decide if it's worth taking the risk to bid on two or more partials.